1. The Field of the Invention
This invention relates generally to apparatus and method for sensing displacements, and more particularly to a highly accurate apparatus and method for measuring translational displacements.
2. The Background Art
Structural health monitoring of objects has been of interest ever since objects or structures such as aircraft, buildings, bridges, etc., were developed. Numerous techniques have been used to determine the point at which the structure will fail. For example, one technique used to monitor the "health" of an object or structure has simply been to keep track of the age, i.e., when a structure gets older, it becomes weaker, thus, approximate life span of a structure has been used to monitor the health of the structure. Another technique to monitor "health" of a structure has been to maintain records of the activities which the structure has performed or been involved with. Yet another technique has been to monitor the size of cracks in the structure. More recently, "health" monitoring has been accomplished through electronic circuitry such as an extensometer sensor.
Extensometer sensors measure relative displacement such as bending, compression, or stretching of a structure. For example, such sensors can be used to detect the load (or strain) of a physical structure or to calculate the number of strain cycles through which a physical structure has passed and at what level of stress each strain cycle induced in the structure. When the sensors are used in this manner, they are commonly referred to as strain sensors or strain gages.
Prior art strain sensors were commonly placed within the physical structure to monitor strain and wear of the structure, from which an estimate of the life expectancy of a structure could be made and safety precautions taken to prevent accidents that might result if the structure were to fail. Information on the life expectancy of a structure is also useful in determining when a physical structure should be repaired or removed from use.
An area where life expectancy estimates would be of special value is in aircraft maintenance. As aircraft fleets age and exceed initial design lifetimes, inspection and maintenance issues become increasingly important. Another area where life expectancy estimates are of particular value is in buildings or bridges. Knowing the existing strength or safety level of buildings and bridges can prevent disasters which may not otherwise be avoided. As such, interest in developing structural monitoring systems has grown significantly in recent years.
One drawback of typical prior art strain sensors is that they are imbedded within a structure to be monitored, and this, of course, weakens the structure. Other drawbacks of typical prior art strain sensors are (i) labor intensive installation requirements, (ii) analog outputs which drift and require frequent calibration, (iii) extensive wiring and shielding requirements, and (iv) single use characteristics.